Genus–differentia Definition - Criteria For Genus-differentia Definitions

Criteria For Genus-differentia Definitions

There are some more or less standard criteria for judging the appropriateness of a genus-differentia definition. These criteria include the following:

• A definition should state the essential attributes of the species.

For instance, a circle is uniquely described as "a planar closed figure enclosing greater area than any other planar closed figure of equal perimeter." However, this definition, while having the appropriate extension, fails to capture the "essence" of circle as a type of planar closed figure. The difference stated here is not the conventional connotation of the term circle.

• A definition must not be circular.

The definiens should not appear in the definiendum, as in the definition, "A compulsive gambler is a person who gambles compulsively."

• A definition must not be too broad.

The difference must be chosen so that every member of the genus with that difference is a member of the species being defined. Thus, the definition, "A bird is an animal with wings," is too broad, since bats (for example) are also animals with wings, and bats are not birds.

• A definition must not be too narrow.

The difference must be chosen so the every member of the species has that property. Thus, the definition, "A bird is a feathered animal that can fly," is too narrow, since ostriches are birds, but they cannot fly.

• A definition must not be expressed in ambiguous, obscure or figurative language.

This catch-all criterion aims at making the definition as precise and easily understood as one can. Thus, technical jargon should be avoided in non-technical contexts and the terms chosen should not be easily misinterpreted. For instance, one may define "faith" as "true belief", but it is unclear whether this definition means "a belief which is truly held" or "a belief which is true," since English allows either interpretation.

One should also avoid figurative or metaphorical definitions, since such definitions tend to obscure rather than elucidate meaning. Thus, the definition of "bread" as "the staff of life" is a poor definition per this criterion.

• A definition should not be negative where it can be affirmative.

As Copi writes, "The reason for this rule is that a definition is supposed to explain what a term means, not what it does not mean." Thus, one prefers to define "drunkard" as "a person who drinks excessively" rather than "a person who is not temperate in drink."